At first glance, HD 189733b sounds almost comforting.<\/p>\n
It is blue. Not merely imagined as blue in an artist\u2019s palette, but measured as blue in visible light. Seen from a distance, NASA says, the planet would look like a deep blue dot, faintly reminiscent of Earth as seen from space.<\/p>\n
That resemblance ends with the colour.<\/p>\n
HD 189733b is not blue because of oceans, cloud-shadowed continents or a breathable sky. It is a gas giant orbiting extremely close to its star, with a dayside atmosphere hot enough to melt silicates and winds powerful enough to carry those particles around the planet. NASA\u2019s 2013 Hubble release put the contrast plainly: the planet orbits 63 light-years away, appears deep blue, and may rain glass sideways in 4,500-mile-per-hour winds<\/a>.<\/p>\n The planet is a useful antidote to a common visual habit. We see blue and think water. In this case, blue means heat, haze and violent atmospheric chemistry.<\/p>\n How Hubble measured a planet\u2019s colour<\/p>\n HD 189733b belongs to a class known as hot Jupiters: large gas planets orbiting very close to their stars. It was discovered in 2005 and has become one of the most studied exoplanets because it transits, meaning it regularly passes in front of and behind its star from our point of view.<\/p>\n That geometry lets astronomers do something subtle. They cannot take a normal photograph of the planet\u2019s surface. Instead, they measure the combined light from the star and planet, then watch what changes when the planet slips behind the star. The missing light is the planet\u2019s contribution.<\/p>\n Using Hubble\u2019s Space Telescope Imaging Spectrograph, astronomers measured the system before, during and after one of those secondary eclipses. The signal was tiny, but it had a colour signature: when the planet disappeared, the system lost more blue light than red or green. The object being hidden was therefore blue.<\/p>\n NASA\u2019s image page for the Hubble result says the telescope measured the planet\u2019s actual visible-light colour and that the deep blue is not caused by oceans, but by a hot atmosphere in which silicate particles can melt into glass-like droplets<\/a> that scatter blue light more efficiently than red light.<\/p>\n That is the key distinction. Earth\u2019s colour is bound up with liquid water, atmosphere and sunlight. HD 189733b\u2019s colour is an atmospheric warning sign.<\/p>\n A blue planet that is not Earth-like<\/p>\n The planet orbits only about 2.9 million miles from its star, far closer than Mercury orbits the Sun. It completes an orbit in just 2.2 Earth days and is tidally locked, so one hemisphere constantly faces the star while the other remains in darkness.<\/p>\n